Recent year, there have been increasing demand for esthetic improvements of teeth such as improvements of contours, alignment and integrity of teeth in dental therapy. In particular, there have been more cases of young women desiring such dental therapy so as to whitening teeth as an important element of beauty. Generally, the cause of dental discoloration and pigmentation or stain falls into the two categories: (1) extrinsic causes such as sedimentation of colored materials (tobacco, tea, etc.), pigment generating bacteria, discoloration of filling materials (primarily composite resins) and metal salts (primarily amalgam, silver nitrite, and ammonia silver); and (2) intrinsic causes such as aging, chemicals or medicine (ex. fluorine and tetracyclines), dysmmetabolism and hereditary, and dental injuries. The National Health Insurance is applicable to bleaching methods for, the latter, discolored teeth intrinsically.
Several methods have been proposed as methods for esthetic improvement of discolored teeth, among which bleaching may be considered as a highly effective method for the preservation of dentine if a suitable method for each decease case would be selected and carried out properly, although there may be some cases of color reversion. Bleaching is basically a method for decolorizing colored materials through a chemical reaction. In the past, there were various reports of bleaching agents comprising a variety of chemicals based on vital bleaching and non-vital bleaching as well as bleaching methods using such agents.
The following are typical examples:
(1) Bleaching method using 30% H2O2 as a chemical combining light and heat in parallel.
This is a method in which strips of gauze soaked in 30% H2O2 are placed on the front surfaces of teeth and are irradiated for 30 minutes from the left and right by using two 500 W photographic lamps. In this method, the lamps are set as close to the gauze as possible and the H2O2 should be replenished about every 5 minutes so as to prevent them from drying.
(2) Bleaching method using 30% H2O2 as a chemical combining high frequency current in parallel.
This is a method in which strips of gauze soaked in 30% H2O2 are placed on the front surface of teeth and high frequency current is delivered for one second with a spoon-shaped tip of a frequency scalpel and is not delivered for 8 second. The operation should be repeated 6 to 8 times and H2O2 should be replenished to keep them wet during the treatment.
(3) Bleaching method using a paste comprising Aerosol (fine silica powder) mixed with 35% H2O2 and the paste is applied.
This is a method in which the paste is applied onto the surfaces of the etched teeth, 15 minutes later the teeth are rinsed with water and then polished. The method gives a highly bleaching result without using light or heat as Aerosol serves as a moisturizing material to prevent the bleaching agent from drying out and also enhances the bleaching effects. The 35% H2O2 should be handled with care due to its highly corrosive property.
(4) Bleaching method using a paste (Shofu Hi-Lite:tradename), obtained by kneading a 35% H2O2 solution and a powder comprising potassium sulfate, manganese sulfate, silicon dioxide or the like, as a chemical.
This is a method in which the paste is placed on the front surfaces of teeth and worked for 10 minutes or irradiated it with light for 3 minutes by applying a visible light ray radiation device. An advantage of the method is as follows: The paste shows a light green immediately after the kneading and then turn yellow by the light irradiation. If the paste turns dark-brown immediately after the kneading, this indicates that the bleaching effects of the liquid have diminished. However there is the same disadvantage mentioned the above due to the use of 35% H2O2.
(5) Bleaching method using a mixture of 1 ml of 30% HCl, 1 ml of 30% H2O2, and 0.2 ml of diethyl ether as chemical (Improved Machines bleaching).
In this method, the mixture is allowed to act for 5 minutes on tooth surfaces, and then the teeth are polished for 15 minutes under light pressure using a polishing disc. The treatment is repeated 3 times, then they are neutralized with 5.25% NaOCl and the teeth are thoroughly rinsed with water (Oral Surg., 26: 871-878(1968), J. Am. Dent. Assoc., 87: 1329(1973)). In this method, since the paste may scatter into the patient's eyes, it is necessary to protect them from it adequately.
(6) Bleaching method using a paste obtained from kneading 30% H2O2 and a sodium perborate powder (Walking bleach method).
In this method, to dilate the dentinal canaliculus and enhance the bleaching effects, the walls in the pulp cavity are treated with phosphoric acid for 1 minute, rinsed with water and dried. Then the paste is introduced into the pulp cavity and temporarily sealed with cement. Although this method is currently, widely applied for clinical purposes to which the National Health Insurance is applicable as a simple and highly effective method. Its disadvantage is the same as that mentioned in (3).
There have been other many bleaching methods reported as follows: The dental bleaching method using the bleaching agent comprising a mixture of aqueous hydrogen peroxide and ortho-phosphoric acid (Japanese Patent Application Laid-Open No. H8-143436/1996). The bleaching agent obtained from mixing silicic acid anhydride with aqueous hydrogen peroxide and the vital bleaching method using the bleaching agent (Japanese Patent Application Laid-Open No. H5-320033/1993). Further, the dental bleaching compositions comprising a dental bleaching agent (hydrogen-urea peroxide, hydrogen peroxide-carbamide, carbamide peroxide, and the like) and a matrix such as carboxymethylen, and a bleaching method using such a dental bleaching agent (Japanese Patent Application Laid-Open No. H8-113520/1996).
On the other hand, bleaching methods and bleaching agents for dental bleaching require the following conditions:                (a) pronounced bleaching results,        (b) none toxicity of bleaching agents        (c) easy operations,        (d) no detraction to dental physical properties after bleaching,        (e) efficacious for vital tooth bleaching as well as non-vital tooth bleaching,        (f) speedy bleaching results, and so on.        
A bleaching method fulfilling the above conditions would be capable of affording aesthetic improvement while keeping dental contour with remarkably improved results. However, in conventional bleaching agents or methods, the primary bleaching agent is 30% to 35% aqueous hydrogen peroxide, which is highly corrosive and its oxidative property is the base for bleaching teeth. Anyhow, all of various bleaching methods carried out currently in Japan should be the combination of 30% to 35% aqueous hydrogen peroxide, various utensils and other chemicals. A bleaching method carried out in U.S.A. has been the use of 10% urea peroxide in lieu of 30% to 35% aqueous hydrogen peroxide, however, this method has been litigated over the problems in terms of efficacy and safety, and has not obtained the governmental approval in Japan.
Taking the aforementioned status of the bleaching methods into the consideration, the inventors of the present invention have found that, without using highly toxic 30% to 35% aqueous hydrogen peroxide, the intended objectives were achieved by the combined use of titanium dioxide having photocatalytic action and low concentration aqueous solution of hydrogen peroxide as efficacious constituents. The bleaching method using the combination shows safer and simpler, and also is remarkably efficacious to vital tooth as well as non-vital tooth (Japanese Patent Application Laid-Open No. H11-92351/1999).